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Originally Posted by DampRobot
Any details on how you implemented your catapult? I'm interested in how you packaged the springs in what looked to be a really tiny space, and how you designed for an optimized shot/designed for adjustability of the shot.
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I don't have any pictures but I will do my best to explain.
We put the pivot point of the throwing arm all the way at the front of the robot. In order to have a reaction point for extension springs (we looked into others but ultimately were most comfortable with extension) the arm is actually an L shape. There is about 600lbs of springs attached to a 9" lever arm. Their were 6 total springs. Two were 2" OD and the other four were 1" OD. To save space we actually put two of the smaller springs down the center of the two larger. Those four springs were attached to a fork towards the back of the robot. One side of the fork is fixed to the robot while the other side could be driven up and down by a lead screw to finely adjust spring tension. The remaining two springs were attached statically to the robot. In order to pull back the catapult we packaged a choo choo mechanism in the back center of the robot. The second link was a loss link made out of dyneema cord that went forward around a pulley then up attaching to the throwing arm. A seat belt strap could be finely adjusted to change the exit angle of the catapult.
We had three co-dependant variables when tuning the shot:
-spring tension (lead screw)
-angle of thrower when choo choo overcentered (dyneema string length)
-exit angle of thrower (seat belt length)
There were a number of things that I think were key to our catapult's success. The first is the enormous amount of potential energy it had stored combined with an incredibly short change in angle. A very short dwell time in the robot meant that other physics - such as the robot moving, being bumped, or hit - did not meaningfully affect the shot. By the end of the season we were also actually shooting through our ball hoop. This meant that the ball really couldn't move around at all within the robot even while firing. Other small things like using springs instead of surgical tubing meant that the shot never changed once we dialed it in even over multiple competitions.
Glad you like the robot, hopefully Jim or someone will pop in with thrower pictures to better explain.
Cheers, Bryan
Edit: Beaten by Andrew -- not fast enough on the draw